There are various methods by which openings are created in a production liner for injecting fluid into a formation. In a “plug and perf” frac job, the production liner is made up from standard lengths of casing. Initially, the liner does not have any openings through its sidewalls. The liner is installed in the wellbore, either in an open bore using packers or by cementing the liner in place, and the liner walls are then perforated. The perforations are typically created by perforation guns that discharge shaped charges through the liner and, if present, adjacent cement.
The production liner is typically perforated first in a zone near the bottom of the well. Fluids then are pumped into the well to fracture the formation in the vicinity of the perforations. After the initial zone is fractured, a plug is installed in the liner at a position above the fractured zone to isolate the lower portion of the liner. The liner is then perforated above the plug in a second zone, and the second zone is fractured. This process is repeated until all zones in the well are fractured.
The plug and perf method is widely practiced, but it has a number of drawbacks, including that it can be extremely time consuming. The perforation guns and plugs are generally run into the well and operated individually. After the frac job is complete, the plugs are removed (e.g., drilled out) to allow production of hydrocarbons through the liner.